Most PowerBuilder developers are aware of object-oriented concepts like inheritance, encapsulation, and polymorphism. A lot has been said about them, and I don't want to reinvent the wheel by discussing what they are and how they're used in PB applications. The main purpose of this article is to explain one of the lesser known concepts of object-oriented programming, namely, upcasting. Later I'll give an example of method and variable overriding in conjunction with upcasting in PowerBuilder and how it differs in Java.

I came across upcasting when I was preparing for Sun certification for Java. When I learned more about it, I realized that it's something common to PowerBuilder, too. In fact, I had used it before, but I didn't know the term. I then asked a few colleagues if they knew about it: these longtime PowerBuilder developer friends had no idea what was I talking about. That's when I decided to write an article about it.

What Is Upcasting ?
Some of you may be familiar with the term casting. In many programming languages it means changing the data type. Casting can be implicit or explicit. For example, consider this code in PowerBuilder:

int li_rowcount
li_rowcount = dw_1.RowCount()

The return type of 'RowCount()' function is long. However, the above assignment is perfectly valid even though the variable 'li_rowcount' is declared 'int' (of course, there will be problems in the runtime if the 'Rowcount()' returns a value greater than the maximum 'Int' value). This is possible because of implicit casting supported by PowerBuilder

Several low- and mid-level languages support explicit casting. Once you understand casting, upcasting should be obvious if you visualize the inheritance tree below.

In this example the user object (or class) 'n_cst_son' is inherited from the user object (or class) from 'n_cst_father'.

By virtue of inheritance, 'n_cst_son' class gets all the functions (in OO terminology, called as methods) and properties of the parent class. But that's not all. Inheritance also defines a 'relation' between child class and parent class. This relation is best described by saying: 'The child class is of type ÔParent' class.'

For example, in the script of your window, the following is perfectly legal:

n_cst_father lnv_father
lnv_father = create n_cst_son

Here the object of child class is created and assigned to the object of type parent class. This is possible because of upcasting by virtue of inheritance.

Examples of Upcasting At this point some of you may be wondering why, how, and where you'd use upcasting in PowerBuilder. Well, you've already used it if you ever passed a DataStore or nonvisual user object through Message object's PowerObjectParm. As you may recall, it's a common practice in PowerBuilder to pass structures and Data-Stores through a Message object. Since all PB objects are inherited from PowerObject, any PB object can be passed.

Upcasting is indispensable if you're writing any framework or even a generic program. Consider this function, which returns the Filter string for both DataWindows and DataStores:

//parameter a_dw_or_ds PowerObject
string ls_filter

ls_filter = a_dw_or_ds.dynamic &
describe("DataWindow.Table.Filter")

// A Question mark means no filter set.
if ls_filter = '?' then
ls_filter = 'Error or NoFilter'
end if

return ls_filter

Listing 1 contains the complete code for the upcasting window.

Overriding Methods and Variables in PB and Java
In the next code we'll see overriding methods and variables in PowerBuilder in conjunction with upcasting and how it's different in Java.

I created two NVOs shown earlier: 'n_cst_son' inherited from 'n_cst_ father'. Each object has a defined one-instance variable for salary and both have a function with an identical signature that returns the age. In essence, the child class has overridden the variable salary as well as the function that returns the age.

On the command button of a window I coded the following

n_cst_father lnv_father
lnv_father = create n_cst_son

Now the question is, what would be the value of lnv.father.i_salary? Of lnv_father.uf_getage()?
Will it return the son's value or the father's value?
Note that 'lnv_father' is declared to be of type 'n_cst_father' but is assigned the reference to 'n_cst_son'.
It turns out that PowerBuilder totally forgets about the declaration of 'lnv_father' once we assign 'n_cst_- son' object to it. Thus both values returned are of 'n_cst_son' (see Figure 1 and Listing 2).

Similar code in Java reveals that Java never forgets the type of the object when looking for instance variables (see Figure 2 and Listing 3).

Summary
To simplify the job of a programmer, PowerBuilder hides many of its OO features. However, learning those concepts will always benefit the PB developer. Though Java is more object-oriented than PowerBuilder, and of course has more publicity about being object-oriented, many people seem to forget that PowerBuilder is object-oriented too, and that many of the same features existed in PowerBuilder long before Java was born.